This invention relates to color picture tubes and particularly to such tubes having a slit type apertured mask.
Shadow mask type color picture tubes usually include a screen of red, green and blue emitting phosphor lines or dots, electron gun means for exciting the screen and a shadow mask interposed between the gun means and the screen. The shadow mask is a thin multiapertured sheet of metal precisely disposed adjacent the screen so that the mask or apertures are systematically related to the phosphor lines or dots.
Color picture tubes having shadow masks with slit shaped apertures have received relatively recent commercial acceptance. One of the reasons for this acceptance is that the percentage of electron beam transmission through the mask can be made higher for a slit-mask, line-screen type of tube than for a circular-apertured mask, dot-screen type tube. Even though the use of a slit mask provides a definite advantage in electron beam transmission, the percentage of electron beam transmission through a slit mask can be increased even further than is practiced in the present art.
In one type of slit shadow mask, the mask has vertically extending slit apertures which are interrupted by a plurality of spaced bridges or webs which provide mechanical rigidity. The presence of these webs, however, has an effect on electron beam transmission and thus on luminescent brightness.
FIG. 1 shows a portion of a prior art shadow mask 10 having slit shaped apertures 12 wherein the slit apertures 12 are arranged in columns and the apertures in each column are separated by webs 14. Each aperture 12 has an elongated shape with curved ends. Generally, the curvature of each end has a radius approximately equal to half the width of an aperture measured in the center of the aperture. Such curved aperture shape at the webs occurs because of the etching process. Apertured masks are formed by first coating a metal sheet with a photosensitive material, exposing the photosensitive material through a photomaster having a desired aperture pattern thereon and thereafter etching the metal sheet to open the apertures. Most prior art photomasters have rectangularly shaped elements at the locations of the intended apertures. Unfortunately, during etching, the shape of the apertures become rounded at the ends of the apertures instead of being formed as rectangles. Because of this rounding, some electron beam transmission is lost in the corners of the apertures as illustrated by the shaded areas 16 of FIG. 2 which shows the shape of a photomaster element 18 superimposed on its resultant aperture 20. It is, therefore, desirable to develop an aperture pattern which, when etched, will form more nearly rectangular apertures thereby permitting increased electron beam transmission.